Gear shift indication devices (generally called gear shift indicators (GSIs)) are conventionally known as disclosed in, for example, Patent Documents 1 and 2 listed below. In a vehicle equipped with a manual transmission, when a gear is being selected which is different from a suitable gear determined from engine load, vehicle speed, and other driving conditions (e.g., a gear capable of improvement in fuel consumption rate), the gear shift indication device recommends the driver to shift to the suitable gear.
This kind of gear shift indication device is applicable not only to vehicles with a typical manual transmission, but also to vehicles with an automatic transmission capable of sequential shift mode (manual gear shift mode) disclosed in Patent Document 3 listed below and to hybrid vehicles with a continuously variable electric transmission mechanism capable of sequential shift mode disclosed in Patent Documents 4 and 5 listed below.
The following will describe an example where a gear shift indication device is applied to a hybrid vehicle.
A known powertrain used in a hybrid vehicle of the above kind may be made of an engine, a first and a second electric motor (motor generator), and a planetary gear train which is part of a power split mechanism, as disclosed in Patent Documents 4 and 5. Specifically, the engine crankshaft is connected to a planetary carrier in the power split mechanism, the first electric motor (first motor generator MG1) is connected to the sun gear, and the second electric motor (second motor generator MG2) is connected to the ring gear via a reduction mechanism (which is made of, for example, the planetary gear train). Drive wheels are connected to the ring gear via a speed reducer and a differential gear set for power transfer.
Accordingly, during regular travel, the driving force (torque) input from the engine to the planetary carrier is split between the ring gear (for drive wheels) and the sun gear (for the first electric motor). The torque split for the ring gear drives the drive wheels as a direct torque (a torque directly transferred from the engine to the drive wheels). On the other hand, the torque split for the sun gear is transferred to the first electric motor so that the first electric motor can generate electric power which in turn drives the second electric motor (generates torque) for assist torque for the drive wheels.
As described above, the power split mechanism functions as a differential mechanism, and the differential operation enables a function as a transmission (continuously variable electric transmission) capable of electrically changing the gear ratio. As a result, a driving force necessary for the drive wheels is achieved, and at the same time, an engine operation state optimized for fuel consumption rate (an operation state on an optimal fuel efficiency operating line detailed later) is obtained.
When the driver operates the shift lever for gear shifting in sequential shift mode in a hybrid vehicle equipped with a gear shift indication device, for example, the rotational speed of the first electric motor is controlled, and a target rotational speed of the engine is adjusted to realize a gear ratio (a gear ratio that equals the ratio of the rotational speed of the engine to the rotational speed of the transmission output shaft) with the operating gears.
In addition, in this kind of hybrid vehicle, a minimum rotational speed for the engine is specified for each gear specified in sequential shift mode. This is to produce a sufficient braking force by restricting decreases of the rotational speed of the engine when, for example, the accelerator opening degree is low or the accelerator is off, and engine friction is utilized so that its resistance can act as engine brake (braking force on the drive wheels). Accordingly, engine brake that is equivalent to the engine brake in a vehicle equipped with a manual transmission can be simulated in a hybrid vehicle. In addition, if the engine brake is effectively used when, for example, driving down a slope, the maneuverability of a vehicle equipped with a manual transmission can be simulated. In this manner, the minimum rotational speed for the engine is specified increasingly high for gears with higher gear ratios (for lower gears) to simulate engine brake that is equivalent to the engine brake in a vehicle equipped with a manual transmission.